In recent years, with the advances in digital technologies employed for electrical apparatuses, large-capacity and nonvolatile memory devices for storing data, such as music, images, and information, are in increasing demand. As one of the measures to respond to such a demand, a nonvolatile memory device which includes, as a memory cell, a nonvolatile memory element having a resistance value that changes in response to a given electrical signal and keeps the resulting state (referred to as the “ReRAM” hereafter) has received attention. Examples of the reasons for the attention include that the configuration of the nonvolatile memory element is relatively simple and thus implemented easily at a high density, and that consistency with a conventional semiconductor process can be easily ensured.
Such a nonvolatile memory element is classified roughly into two kinds according to a material used for a variable resistance layer (a variable resistance material). One kind is a variable resistance nonvolatile memory element in which a perovskite material (such as Pr1−xCaxMnO3 (PCMO), La1−xSrxMnO3(LSMO), or GdBaCoxOy (GBCO)) disclosed in, for example, Patent Literature 1 is used as the variable resistance material.
The other kind is a variable resistance nonvolatile memory element which uses a binary metal oxide as the variable resistance material. As compared with the aforementioned perovskite material, the composition and structure of the binary metal oxide are extremely simple. On this account, the composition control and film formation at the time of manufacturing can be easily implemented. In addition, because of the advantage of relatively excellent consistency with the semiconductor manufacturing process, a great deal of research have been conducted.
The physical mechanism of a resistance change is still unknown in many respects. However, through the recent research, change in the defect density of conductive filaments formed in the binary metal oxide by oxidation-reduction is regarded as the most likely factor responsible for a resistance change (see Patent Literature 2 and Non Patent Literature 1, for example).
FIG. 19 is a cross-sectional diagram showing a configuration of a conventional nonvolatile memory element 1800 disclosed in Patent Literature 2.
By the application of a voltage (an initial breakdown voltage) between a first electrode 1803 and a second electrode 1806 in an archetypal structure ((a) of FIG. 19) where a variable resistance layer 1805 comprising a metal oxide is positioned between the first electrode 1803 and the second electrode 1806, filaments 1805c are formed ((b) of FIG. 19) to be a current path between the first electrode 1803 and the second electrode 1806 (in the current path, the current flowing between the first electrode 1803 and the second electrode 1806 is locally high in density).